For a number of years, the Running Department of the GWR required a '43xx' type engine with a leading bogie and standard No. 1 boiler. Although the Mogul class were successful, it was felt that a 4-6-0 engine would reduce nosing and provide the ideal all-round locomotive. Such a machine was outlined in Churchward's standardisation plan, although he felt that a ten-wheeled engine would have greater traction if it was to the 2-8-0 configuration, to which he designed the '28xx' class.

When C. B. Collett became Chief Mechanical Engineer in 1922, he was asked to produce a 4-6-0 to replace the Mogul '43xx' class. His answer was to reduce the coupled wheel diameter of 'Saint' class No. 2925 Saint Martin from 6 feet 8½ inches to 6 feet 0 inches. After a short period of evaluation, the first order for eighty locomotives to these dimensions was given. These first eighty were fitted with spring compensating beams fitted between the axles but subsequent orders did not have them fitted and on the original eighty, the beams were gradually removed. Further locomotives of the class were ordered with very little alteration to the original design. Nos. 6916 to 6970 entered traffic without names during the Second World War but naming of these engines took place between 1946 and 1948. Experiments with electric lighting and mechanical lubricators were performed, the latter fitted as standard from No. 7910 onwards.

A series of modifications to the frames, cylinders and saddle were designed by F. W. Hawksworth when he became Chief Mechanical Engineer and this version of the 'Hall' class is known as the '6959' or 'Modified Hall' class. At the same time, larger three-row superheaters were fitted ( except Nos. 6966 to 6970 ) although with subsequent boiler changes, some members of this class received earlier two-row superheater boilers. Altogether, the 'Hall' and 'Modified Hall' classes numbered 330 locomotives.

The power classification of the class was 5-MT, the G.W.R. power class was D and the route availability was RED.

Detail Alterations. Eleven members of the class were converted to oil burning in 1946/7 but all were reverted back by April 1950.